In the fabrication of semiconductor integrated circuit (IC) devices, hundreds of fabrication steps must be performed on a semi-conducting substrate in order to complete the fabrication of such devices. The hundreds of processing steps may include cleaning, deposition, etching, buffer coating and various other necessary steps. In these fabrication steps, a variety of process chemicals, including liquids and gases must be used in different processing machines and be transported from their storage tanks to the machines. A large number of these process liquids are of high viscosity and short shelf life and therefore their transportation between a reservoir and a process machine must be carefully controlled. Deterioration or premature reaction of these process liquids can result in poor quality products and unnecessary machine down time which in turn lead to a decrease in process yield.
One of the more difficult to transport process liquid is a polyimide material that is frequently used as a moisture barrier or a buffer coating. Polyimide has also been used as a popular material to provide .alpha.-particle protection for memory circuits. It is known that soft errors in memory circuits are frequently caused by .alpha.-particles emanating from packaging materials used on the circuits. The .alpha.-particles are generally believed to be emitted by the decay of uranium and thorium atoms which are contained as impurities in the packaging materials for the IC devices. In recent years, decreasing device design rules make IC circuits more sensitive to the .alpha.-particle emission problem. Since .alpha.-particles have low penetrating power in a solid material, polyimide has been successfully used as .alpha.-absorbing coatings on memory devices.
Polyimides belong to a class of organic polymeric compounds that are derived from imidization reactions of amines and organic acids. The polyimides can be used in a similar manner as spin-on-glass materials by spinning and curing a highly viscus compound to produce a planar surface on a wafer for passivation or for buffer coating. It has been broadly used in recent years, aside from its .alpha.-particle absorbing characteristics, to realize its benefits of ease of deposition, flexibility in composition, planarity as a spun film, good temperature endurance, excellent weathering and mechanical wear, low pin hole density and dielectric constant, and low absorption of water when compared to a spin-on-glass material for passivation. Polyimide films are successfully used as inter-level dielectrics and protective overcoating layers on top of an IC device. When used as a protective overcoating layer, it is normally deposited to a thickness of between about 50 .ANG. and about 1,000 .ANG.. As a protective overcoating layer, polyimide further has the benefit of acting as a stress relief layer when covered by a plastic molding compound in a packaging operation.
A conventional method for transporting a process liquid by pneumatic pressure alone is shown in FIG. 1. The liquid dispensing system 10 consists mainly of a supply tank 12 which is normally constructed of stainless steel for chemical resistance and mechanical strength. A liquid container 16 is positioned inside the supply tank 12 with its top open to the cavity 14 of the tank. A high pressure dry air 18 is fed into cavity 14 through a first conduit 20 controlled by a valve 22 connected to a vent 24. Liquid 26 having a top surface 28 exposed to the high pressure dry air is pushed into a second conduit 32 through a second valve 34 to an outlet 36 connected to a nozzle (not shown) for dispensing. The second valve 34 is further connected to a drain pipe which can be used to drain the process liquid 26 when bubbles are observed in the liquid flow.
The liquid dispensing system 10 is operated by pneumatic pressure only and no pump means is employed. As such, even though it has the benefit of a less complicated hardware requirement, it is difficult to accurately control the quantity of liquid it dispenses. Furthermore, the pneumatic pressure system generates air bubbles easily in the liquid 26 which reduces the system efficiency. More defects can be generated when more bubbles are introduced during a replacement of an empty bottle. There is no effective means for controlling the bubble formation or disposing of the bubbles from the system contained in apparatus 10.
Furthermore, when the supply bottle 16 is empty, an operator must replace with a new bottle and the operation of the coating machine must be stopped during such replacement. The efficiency of the process is therefore suffered due to the down time of the machine. Another disadvantage of the pneumatic feed apparatus is that in order to get rid of the bubbles in the liquid flow, a large volume of liquid must be drained. It is known that most processing liquids used in the semiconductor processing industry are very expensive, the draining of large quantity of liquids in order to eliminate bubbles therefore involves a large and undesirable expense. Even with the large volume draining of liquid that contains bubbles, micro-bubbles are still in existence and are normally small enough to go through dispensing nozzles and be deposited on top of a substrate causing the formation of a poor quality coating.
In another conventional method for dispensing process liquid, as shown in FIG. 2, a pump feed apparatus is utilized. The dispensing apparatus 40 consists of a supply tank 42 which is normally constructed of a chemical resistant material such as stainless steel. In the cavity 44 of the supply tank 42, a liquid container 46 for holding a process liquid 48 is positioned. A high pressure dry air supply 50 is used to pressurize the cavity 44, and through an open top in the liquid container 46, to pressurize the surface 52 of the liquid 48 contained in the container 46. The high pressure dry air 50 is supplied to the cavity 44 through a first conduit 54 controlled by a valve 56 which is also vented through a vent 58 to the atmosphere. A second buffer tank 60 which may also be constructed of a chemical resistant material such as stainless steel is used to store liquid 62 transported from the first container 46.
The transportation of liquid between the supply tank 42 and the buffer tank 60 is achieved by a second conduit 64 which is in fluid communication with both containers 46 and 60. The buffer tank 60 is further equipped with a vent 66 and an outlet conduit 68 for feeding to a pump 70. The pump 70, is most likely electrically operated moves liquid 62 through a pumping action through a filter 72 and conduit 74 to a dispensing nozzle (not shown). The pumping apparatus 40 shown in FIG. 2 solves the deficiencies of the pressure feed apparatus 10 shown in FIG. 1 by providing a more accurate flow rate control or quantity of liquid delivered by the dispensing nozzle.
Furthermore, by using an intermediate tank, i.e., the buffer tank 60, the process machine need not be shut down when the first container bottle 46 empties and must be replaced. However, there are several disadvantages of the pump feeding apparatus 40. For instance, the pump feeding apparatus 40 is complicated in its construction and therefore is difficult to maintain. Furthermore, since a stepping motor is used for the pump, there is always a small chamber space in the pump that acts as a dead space cumulating a small pool of the pumped liquid. When the pumped liquid has a short shelf life, such as that of a polyimide overcoating material, a highly viscus gel is produced by a premature reaction (or polymerization) in the dead space. The gel formation causes contamination problems and poor accuracy of the pumping action which requires the pump be serviced more frequent than its regular periodic maintenance schedule required. The maintenance cost for a pump feed apparatus 40 is therefore substantially higher than desired.
It is therefore an object of the present invention to provide a liquid dispensing system that does not have the drawbacks or shortcomings of the conventional liquid dispensing systems.
It is another object of the present invention to provide a liquid dispensing system that does not require the operation of a pump for dispensing such liquid.
It is a further object of the present invention to provide a liquid dispensing system that utilizes a differential pressure between a supply tank and a buffer tank, and gravity of the liquid for dispensing through a dispensing nozzle.
It is another further object of the present invention to provide a liquid dispensing system used in semiconductor processing machines that is especially suitable for dispensing liquids that have high viscosity and short shelf life.
It is still another object of the present invention to provide a liquid dispensing system that is equipped with a conduit in fluid communication with a first container and a second container wherein the conduit is further equipped with a bubble sensing means and a bubble release valve.
It is yet another object of the present invention to provide a liquid dispensing system for use in semiconductor processing wherein a buffer tank made of a material that is permeable to air but not to liquid is utilized to prevent the gel formation in the liquid dispensed.
It is still another further object of the present invention to provide a liquid dispensing system for use in semiconductor processing machines that utilizes a dispensing container that has an inclined bottom for easy feeding of the liquid to a dispense nozzle by gravity.
It is yet another further object of the present invention to provide a liquid dispensing system for use in semiconductor processing machines wherein the pneumatic pressure in the first supply tank is higher than that in the second buffer tank such that liquid can be fed through a conduit to a dispensing nozzle.
It is still another further object of the present invention to provide a method for dispensing a liquid to a semiconductor processing machine by utilizing a differential pressure between a supply tank and a buffer tank such that air bubble formation in the liquid can be minimized and the liquid can be fed to a dispensing nozzle by the differential pressure and gravity of the liquid without using a pump.
It is still another further object of the present invention to provide a liquid dispensing system utilizing an inclined and sharp discharging tip of a conduit to reduce bubble retention in a buffer tank.